Article pubs.acs.org/accounts
Sustainable Elastomers from Renewable Biomass Zhongkai Wang,*,†,‡,§ Liang Yuan,‡,§ and Chuanbing Tang*,‡ †
School of Forestry and Landscape Architecture, Anhui Agriculture University, Hefei, Anhui 230036, China Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208, United States
‡
CONSPECTUS: Sustainable elastomers have undergone explosive growth in recent years, partly due to the resurgence of biobased materials prepared from renewable natural resources. However, mounting challenges still prevail: How can the chemical compositions and macromolecular architectures of sustainable polymers be controlled and broadened? How can their processability and recyclability be enabled? How can they compete with petroleum-based counterparts in both cost and performance? Molecular-biomass-derived polymers, such as polymyrcene, polymenthide, and poly(ε-decalactone), have been employed for constructing thermoplastic elastomers (TPEs). Plant oils are widely used for fabricating thermoset elastomers. We use abundant biomass, such as plant oils, cellulose, rosin acids, and lignin, to develop elastomers covering a wide range of structure−property relationships in the hope of delivering better performance. In this Account, recent progress in preparing monomers and TPEs from biomass is first reviewed. ABA triblock copolymer TPEs were obtained with a soft middle block containing a soybean-oil-based monomer and hard outer blocks containing styrene. In addition, a combination of biobased monomers from rosin acids and soybean oil was formulated to prepare triblock copolymer TPEs. Together with the above-mentioned approaches based on block copolymers, multigraft copolymers with a soft backbone and rigid side chains are recognized as the first-generation and second-generation TPEs, respectively. It has been recently demonstrated that multigraft copolymers with a rigid backbone and elastic side chains can also be used as a novel architecture of TPEs. Natural polymers, such as cellulose and lignin, are utilized as a stiff, macromolecular backbone. Cellulose/lignin graft copolymers with side chains containing a copolymer of methyl methacrylate and butyl acrylate exhibited excellent elastic properties. Cellulose graft copolymers with biomass-derived polymers as side chains were further explored to enhance the overall sustainability. Isoprene polymers were grafted from a cellulosic backbone to afford Cell-g-polyisoprene copolymers. Via cross-linking of these graft copolymers, human-skin-mimic elastomers and high resilient elastomers with a well-defined network structure were achieved. The mechanical properties of these resilient elastomers could be finely controlled by tuning the cellulose content. As isoprene can be produced by engineering of microorganisms, these elastomers could be a renewable alternative to petroleum products. In summary, triblock copolymer and graft copolymer TPEs with biomass components, skin-mimic elastomers, high resilient biobased elastomers, and engineering of macromolecular architectures for elastomers are discussed. These approaches and design provide us knowledge on the potential to make sustainable elastomers for various applications to compete with petroleum-based counterparts.
1. INTRODUCTION
Elastomers have found versatile applications such as adhesives, coatings, tubes, tires, and fibers.11 Elastomers can be divided into thermoset elastomers and thermoplastic elastomers according to the types of cross-linking in manufacturing processes. In contrast to the commercial success of biomass-derived plastics, renewable biobased elastomers are still in the infancy stage, although increasing attention has particularly been paid to the biomedical field.12 Molecular biomass, including terpenes, fatty acids, plant oils, etc., has been widely explored in designing elastomers. Sarkar and Bhowmick13 synthesized an elastomer from β-myrcene via emulsion polymerization and elucidated the structure of polymyrcene
Sustainable polymeric materials from renewable biomass could reduce our dependence on fossil fuels and mitigate their adverse environmental effects.1−3 Most renewable biomass falls into two categories: natural polymers and molecular biomass.4 Natural polymers, such as cellulose, hemicellulose, lignin, starch, protein, and chitin, have been utilized in composites, plastics, biofuels, and other applications.5 The development of synthetic polymers from molecular biomass, such as plant oils, rosin acids, terpenes, terpenoids, and furan, is a fast-growing area.6−10 Commercial successes of biobased plastic materials include but are not limited to “Ingeo PLA” from Natureworks, “I’m Green Polyethylene” from Braskem, and “Plant Bottle PET” from Coca Cola. © 2017 American Chemical Society
Received: April 26, 2017 Published: June 21, 2017 1762
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Scheme 1. Representative Chemical Structures and Sources of Plant Oils, and Multifunctional and Monofunctional Monomers from These Oils
(PMYR), which has a glass transition temperature (Tg) of −70 °C. A sustainable thermoset elastomer was further developed through copolymerization of β-myrcene with styrene and subsequent vulcanization.14 The groups of Hillmyer and Hoye used another terpene-derived monomer, α-methyl-p-methylstyrene (AMMS), to replace styrene and prepare sustainable ABA triblock copolymers PAMMS-b-PMYR-b-PAMMS through cationic polymerization. These novel copolymers showed respectable mechanical strength and elasticity compared with styrenic TPEs.15 Biobased lactones have been used for TPEs.16 The Hillmyer group pioneered the use of lactones for constructing various TPEs. They explored poly(β-methyl-δ-valerolactone), polymenthide, and poly(ε-decalactone) as soft segments.17,18 We have taken on the challenge of preparing sustainable elastomers from renewable biomass for correlating their properties with chemical structures. In this Account, we highlight the recent progress mostly in our group on the development of different generations of TPEs containing various levels of biomass components. The first-generation ABA triblock copolymer TPEs from plant oils exploited by many other groups are also included. We also introduce a new generation of TPEs that employ a concept combining a rigid polymer backbone (e.g., cellulose) and soft grafted side chains. In addition, other types of biobased functional elastomers are discussed.
structure of triglycerides and several multifunctional derivatives. Thermoset materials have been prepared from these multifunctional monomers. To make reprocessable thermoplastic elastomers, monofunctional monomers are needed for integration into a copolymer system. However, conversion of plant oils into monofunctional monomers for TPE applications has been a challenge. 2.1. Monofunctional Monomers and Homopolymers from Plant Oils
One early example of monofunctional monomers from plant oils is the synthesis of an oxazoline monomer, SoyOx, from soybean oil (Scheme 1), which was reported by the Henkel Corporation.19 Fatty acids from soybean oil were converted into SoyOx through a two-step condensation reaction with ethanolamine. Hoogenboom and Schubert20 carried out microwave-assisted cationic polymerization of this monomer and examined the polymerization kinetics. The bulk polymerization was a living process, and full conversion was rapidly achieved with the double bonds unaffected. Chisholm and coworkers prepared a vinyl ether monomer, 2-VOES, by transesterification between ethylene glycol vinyl ether and soybean oil.21 By the use of 1-isobutoxyethyl acetate as an initiator and ethylaluminum sequichloride as a co-initiator, a viscous polymer with a Tg of −92 °C was prepared.22 Since both oxazoline and vinyl ether monomers were polymerized by living cationic polymerization, which requires quite strict reaction conditions and high purity of monomers, strategies toward polymerizable monomers under less stringent conditions were sought. Very recently, the Voronov group reported a vinyl monomer obtained by transesterification between Nhydroxyethyl acrylamide and soybean oil.23 A homopolymer with a Tg of −6 °C was prepared by free radical polymerization. Copolymerization with styrene, methyl methacrylate, and vinyl
2. PLANT OILS FOR THERMOPLASTIC ELASTOMER APPLICATIONS Plant oils are available worldwide at relatively low cost that is competitive with petroleum-based chemicals. The multifunctional structures of their major components, triglycerides, are ideal precursors for making thermoset materials with the formation of cross-linked networks. Scheme 1 gives a general 1763
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Scheme 2. (A) Synthesis and Polymerization of (Meth)acrylate Monomers from High-Oleic Soybean Oil (HOSO); (B) Amino Alcohols Used for Making SBOHsa
a
Reproduced with permission from ref 26. Copyright 2016 The Royal Society of Chemistry.
100%. Sixteen (meth)acrylate monomers were then prepared via (meth)acrylation of SBOHs. These monomers were subjected to simple free radical polymerization to provide various polymers. A structure−property relationship of the 16 homopolymers was established with the aid of thermal and mechanical analysis. Six polymers from monomers containing tertiary (3°) amide exhibited Tg’s from −6 to −54 °C, while 10 polymers from monomers with secondary (2°) amide had Tg’s from 20 to 60 °C (Figure 1).26 The main scenario behind such a dramatic difference in thermal properties is the existence of hydrogen bonding in 2° amide-containing polymers, while it is absent in polymers containing 3° amides. Flexible thermoplastic
acetate was carried out to obtain polymers with 20−80 wt % soy-based acrylic units. This approach was also applicable to sunflower oil, linseed oil, and olive oil.24 Although (meth)acrylate monomers have been prepared from plant-oil-derived fatty acids and fatty alcohols, the preparation of these precursors normally involves hydrolysis, hydrogenation, and fractionalization. We recently reported a new strategy to make (meth)acrylate monomers from higholeic soybean oil (HOSO).25,26 As shown in Scheme 2, 17 hydroxyl-capped fatty amides (SBOHs) were first obtained by an amidation reaction between HOSO and different amino alcohols. The conversions of most reactions were as high as 1764
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Figure 1. (A) A series of soybean-oil-derived (meth)acrylate polymers with varied Tg’s. (B) Illustration of microstructures involving 2° and 3° amides in polymers. (C) Physical appearances of thermoplastic and viscoelastic polymers. Reproduced with permission from ref 26. Copyright 2016 The Royal Society of Chemistry.
Figure 2. (A) Synthesis and polymerization of norbornene monomers from SBOHs. (B) Differential scanning calorimetry (DSC) curves of norbornene polymers.
Scheme 3. (A) Preparation of the ABA Triblock Copolymer PSt-b-PSBA-b-PSt by ATRP; (B) Illustration of Phase Separation of This ABA Triblock Copolymer
films could be obtained, which showed strain at break of 120− 280% and stress at break of 1.2−3.1 MPa. The relatively low
tensile strength is largely due to poor chain entanglements, as described in the next section. 1765
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Figure 3. (A) DSC curves of PSBA and triblock copolymers. (B) Tensile plots of PSt-b-PSBA-b-PSt with different fractions of PSt blocks.
Figure 4. Light cross-linking approach to improve chain entanglement: bis-TAD click reaction of the PSBA middle block in the triblock copolymer PSt-b-PSBA-b-PSt. Reproduced with permission from ref 29. Copyright 2015 The Royal Society of Chemistry.
Three SBOHs were epoxidized and esterified with exo-5norbornenecarboxylic acid to afford norbornene monomers (Figure 2A).25 Homopolymers prepared by ring-opening metathesis polymerization (ROMP) showed viscoelastic properties with Tg’s between −27 and −33 °C (Figure 2B). The correlation between the structures of HOSO-derived monomers and the properties of their homopolymers provides useful guidance for designing sustainable elastomers, which is elaborated below.
copolymers of lauryl acrylate (LAc) and stearyl acrylate (SAc) as the soft block.27 The triblock copolymers polystyrene-bpoly(LAc-co-SAc)-b-polystyrene exhibited TPE properties, which were influenced by the LAc/SAc ratio and the overall weight fraction of poly(LAc-co-SAc). In another recent report,28 lauryl methacrylate (LMA) was used to make the soft block and a methacrylate monomer from plant-derived salicylic acid (ASEMA) was utilized to make the hard blocks, resulting in PASEMA-b-PLMA-b-PASEMA. Chisholm and coworkers prepared ABA triblock copolymers containing 2-VOES and cyclic hexane vinyl ether (CHVE) by cationic polymerization.21 Two distinct Tg’s (42 and −95 °C) were clearly observed from PCHVE-b-P(2-VOES)-b-PCHVE, indicating phase separation between PCHVE and P(2-VOES). However, mechanical properties of these polymers were not reported. As discussed above, HOSO-derived (meth)acrylate monomers with 3° amides lead to viscoelastic polymers with Tg’s in the range of −54 to −6 °C.26 These polymers could be used as a soft middle block to prepare ABA triblock copolymer TPEs. For example, a homopolymer from a soybean acrylate monomer, SBA (Scheme 3A), has a Tg of −33 °C and is fluidlike at room temperature. ABA triblock copolymers were
2.2. TPEs from Plant-Oil-Derived Monomers
The most traditional TPEs are ABA triblock copolymers with a soft middle block and hard end blocks. The hard blocks phaseseparate into nanoscale domains dispersed in a soft matrix and act as physically cross-linked junctions. Polystyrene-b-polybutadiene-b-polystyrene (SBS), polystyrene-b-polyisoprene-bpolystyrene (SIS), and polystyrene-b-poly(ethylene−butylene)b-polystyrene (SEBS) are widely used engineering TPEs. Current academic efforts have been relatively focused on using biomass-derived monomers to make the soft components of TPEs, while a few biomass-based monomers have been also reported to make rigid blocks. Robertson and co-workers reported renewable thermoplastic elastomers with random 1766
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Scheme 4. Preparation of Diblock Copolymers PSBMA-b-PFMA and PSBMA-b-PDAEMA Using Biomass-Derived Monomers through Metal-Free ATRP
higher. Thus, the entanglement between PSBA chains in the triblock copolymers is highly suppressed, as its molecular weight is only 32 kg/mol. To improve the mechanical properties, there are a few strategies to overcome the poor chain entanglement.31,32 We introduced light cross-linking between the soft PSBA blocks in the hope of improving the chain entanglement while maintaining the processability of the triblock copolymers (Figure 4). We utilized triazolinedione (TAD)-based click chemistry, as recently introduced by the Du Prez group.33 The coupling reagent was bis-TAD, which can effectively react with double bonds in fatty side chains without the use of any catalysts. As shown in Figure 4, when 5% of the double bonds in PSBA were cross-linked by bis-TAD, a solution of a selected triblock copolymer (PSt49-b-PSBA82-bPSt49) formed a free-standing gel after 1 h. The polymer maintained solubility when only 1% of the double bonds were cross-linked. The tensile strength of the product increased from 0.9 to 1.8 MPa after 1% cross-linking, while that of the 5% cross-linked sample increased to 6.2 MPa. To increase the biomass content, block copolymers containing a soybean-oil-based methacrylate monomer, SBMA, as a soft block and a rigid block from either plantbased furfural methacrylate (FMA) or rosin-based dehydroabietic ethyl methacrylate (DAEMA) were prepared through ATRP (Scheme 4).34 Light-induced metal-free ATRP was utilized to avoid using metal catalysts.35,36 Ethyl α-bromophenylacetate (EBPA) and 10-phenylphenothiazine (PTH) were used as an initiator and an organic photocatalyst, respectively. PSBMA with predictable molecular weight and narrow molecular weight distribution could be achieved under UV or visible-light irradiation. Kinetic studies further confirmed a “living” radical polymerization behavior. The end-group fidelity
prepared by atom transfer radical polymerization (ATRP) with PSBA as the middle block and polystyrene (PSt) as the rigid outer block.29 Ethyl bis(2-bromoisobutyrate) was used as a symmetric initiator for the polymerization of SBA. The macroinitiator Br-PSBA-Br was chain-extended with styrene. With a constant PSBA middle block at a repeat unit of 82, the length of each outer PSt block was varied from 49 to 70, 102, and 118 to tune the PSt weight fraction from 27.4% to 32.5%, 40.1%, and 49.4%. These triblock copolymers displayed two distinct Tg’s as observed by DSC (Figure 3A), implying a micro-phase-separated morphology (Scheme 3B). With increasing chain length (and thus weight fraction) of PSt, the Tg of the PSBA matrix slightly increased (from −26 to −21 °C). However, Tg of the PSt phase exhibited an appreciable increase from 67 to 100 °C. This could be explained by partial chain mixing between PSt and PSBA, especially when the weight fraction of PSt is low. Monotonic tensile profiles for these block copolymers are given in Figure 3B. With increasing PSt weight fraction, the ultimate tensile strength increased from 0.9 to 6.6 MPa, while the strain at break was maintained at nearly 100%. Block copolymers with 27.4% and 32.5% PSt showed elastomeric properties, while those with higher PSt content displayed more thermoplastic behaviors. It needs to be mentioned that the tensile strength of these elastomers is still significantly lower than those of commercial SBS and SIS. We attribute this phenomenon to the poor chain entanglement from the soft middle block, which is crucial for the mechanical strength of ABA triblock copolymers. The chain entanglement molecular weight of PLMA with a 12-carbon alkyl side group was reported to be 225 kg/mol.30 This specific molecular weight for PSBA with a 20-carbon alkyl side chain is expected to be even 1767
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Accounts of Chemical Research Scheme 5. Preparation of the ABA Triblock Copolymer PDAN-b-PSBN-b-PDAN through ROMP
Figure 5. (A) DSC and (B) tensile plots of PDAN50-b-PSBN105-b-PDAN50, PDAN25-b-PSBN105-b-PDAN25, and PDAN50-b-PSBN210-b-PDAN50. The numbers indicate the repeat units.
Scheme 6. Illustration of Architectures I, II, and III for Making TPEsa
a
Adapted from ref 37. Copyright 2013 American Chemical Society.
evaluate whether these “hard” and “soft” biobased monomers could be used to make elastomeric materials. The relatively low elongation (∼100%) of PSt-b-PSBA-b-PSt samples (Figure 3B) might be attributed to the low elasticity of the PSBA polymer backbone, since a long pendant chain is present at each repeat unit with only two bonds separated from each other on the backbone. The elasticity could be enhanced
of PSBMA was corroborated through chain extension with FMA or DAEMA to reach the diblock copolymers PSBMA-bPFMA and PSBMA-b-PDAEMA. Two distinct Tg’s detected by DSC confirmed the presence of phase separation. The Tg’s of PSBMA, PFMA, and PDAEMA were found to be 2, 62, and 77 °C, respectively. Further development is particularly needed to 1768
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Accounts of Chemical Research Scheme 7. Synthesis of Cell-g-P(MMA-co-BA) and Cell-g-P(LMA-co-DAEMA) by “Grafting from” ATRPa
a
Adapted from ref 37. Copyright 2013 American Chemical Society.
cellulose−isobutyryl bromide (Cell-Br). “Grafting from” ATRP was carried out to prepare Cell-g-P(MMA-co-BA). DSC results showed that the Tg’s of side chains can be finely tuned in the range of −46 to 128 °C by controlling the feed ratio of MMA to BA. Finally, a wide spectrum of TPEs can be obtained that are otherwise challenging to achieve by Architectures I and II. The results showed that Cell-g-P(MMA-co-BA) copolymers have much higher tensile strength and similar strain at break compared to linear P(MMA-co-BA) copolymers. Moreover, Cell-g-P(MMA-co-BA) copolymers with Tg’s around 10 °C showed excellent elastic properties. The simple synthetic method toward novel architectures could be applied to different monomers grafted from various rigid substrates. Recently the Bates group extended this concept to prepare block copolymers grafted from a rigid substrate, despite their focus on thermoplastics.38,39
by further separating the pendant fatty chains. As shown in Scheme 5, triblock copolymers with rigid outer blocks from a dehydroabietic acid-derived monomer, DAN, and a soft middle block from a soybean oil-derived monomer, SBN, were prepared by ROMP through sequential monomer addition. Full conversion of the added monomers can be achieved within 30 min.31 The fatty chains in these triblock copolymers are six bonds away from each other, which could increase the flexibility of the polymer backbone. On the other hand, ROMP allowed the preparation of polymers with much higher molecular weights, making it possible to improve the chain entanglement. After the phase separation was confirmed by DSC (Figure 5A), atomic force microscopy, and small-angle X-ray scattering, the mechanical behaviors of three samples were measured (Figure 5B). The thermoplastic elastomer PDAN25-b-PSBN105-bPDAN25 achieved an elongation at break of 258.5 ± 26.5%, demonstrating a significant improvement from PSt49-b-PSBA82b-PSt49. The two different block copolymers have similar weight fractions of the outer block (∼28%). Cyclic tensile tests proved the excellent elastic recovery of these new TPEs (nearly 100% after the first cycle).
3.1. Renewable Monomers for Multigraft TPEs
We developed a series of (meth)acrylic monomers from rosin and studied their living radical polymerization behaviors.40 By “grafting from” ATRP, we constructed cellulose-graf t-rosin copolymers, which combined cellulose and rosin in one polymer system. However, because of the rigidity of the rosin-based side chains, these graft copolymers are not TPEs.41 We further prepared cellulose-g-poly(lauryl methacrylate-codehydroabietic ethyl methacrylate) (Cell-g-P(LMA-coDAEMA)) via “grafting from” ATRP (Scheme 7). In these graft copolymers, cellulose, LMA, and DAEMA were derived from wood pulp, fatty acid, and rosin, respectively. The Tg’s of P(LMA-co-DAEMA) side chains can be precisely tuned in the range of −50 to 60 °C. Typical stress−strain curves of Cell-gP(LMA-co-DAEMA) suggested that these graft copolymers are good candidates for TPE applications.41 In principle, the rigid backbone is not limited to cellulose but could also be linear/nonlinear rigid polymer chains or small rigid nanoobjects. Lignin, one of the most abundant natural resources, was used as a rigid substrate in Architecture III.41 Lignin macroinitiators (lignin-Br) were prepared by esterification between hydroxyl groups of lignin and 2-bromoisobutyryl bromide. Lignin-Br with different densities of initiators was prepared by controlling the feed ratio of 2-bromoisobutyryl bromide to lignin. Similar to Cell-Br, Lignin-Br was used as a macroinitiator to perform “grafting from” ATRP of MMA and
3. MULTIGRAFT ELASTOMERS WITH A RIGID BACKBONE Two conventional architectures are used for constructing TPEs. As shown in Scheme 6, Architecture I represents ABA triblock or star block copolymers with soft middle blocks and hard end blocks. Architecture II represents multigraft copolymers with a soft polymer backbone and grafted glassy side chains. However, the preparation of these triblock and multigraft copolymers requires stringent conditions, and most of them are nonsustainable.19 Efforts have been devoted to exploring new architectures toward sustainable TPEs. We have designed novel multigraft copolymers with a rigid polymer backbone and soft side chains. The new architecture (Architecture III in Scheme 6) is the reverse of Architecture II. As new-generation TPEs based on Architecture III, cellulosegraf t-poly(methyl methacrylate-co-butyl acrylate) (Cell-g-P(MMA-co-BA)) copolymers were prepared. Cellulose functions as a rigid backbone, and P(MMA-co-BA) performs as an elastic matrix (Scheme 7).37 Cellulose was first dissolved in the ionic liquid 1-allyl-3-methylimidazolium chloride (AMIMCl) and then modified with 2-bromoisobutyryl bromide to obtain 1769
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Scheme 8. (A) Synthesis of Cell-g-PI Copolymers by ARGET ATRP; (B) Self-Assembly of Cell-g-PI Copolymers into a TwoPhase Morphology, As Shown by a DMA Curve of the Derivative of log E′ (E′ = Storage Modulus) as a Function of Temperaturea
a
Reproduced with permission from ref 42. Copyright 2014 The Royal Society of Chemistry.
Figure 6. (A) Illustration of microstructures, (B) step-cycle stress−strain curves, and (C) resilience of HREs during the step-cycle tensile deformations. For clarity in (B), the tensile curves have been shifted along the x axis, and the final strains are given at the top of each plot. Adapted from ref 34. Copyright 2016 American Chemical Society.
BA to obtain Lignin-g-P(MMA-co-BA).25 As we demonstrated with Cell-g-P(MMA-co-BA), the graft copolymers act as typical TPEs when the P(MMA-co-BA) side chains exhibit Tg’s around 10 °C. Lignin-g-P(MMA-co-BA) copolymers with Tg’s in the range from −3.7 to 35.9 °C were prepared. Monotonic stress− strain curves showed that the introduction of lignin into P(MMA-co-BA) improved the mechanical stress, though the elastomeric properties were mediocre. Further optimization of the structure−property relationship is needed. Surprisingly, the results also showed that these Lignin-g-P(MMA-co-BA) copolymers exhibited distinct UV absorption, even with only 0.2 wt % lignin integrated. These lignin-based materials may find applications as coatings that can protect the substrates underneath by blocking UV light exposure.
electron transfer atom transfer radical polymerization (ARGET ATRP) using Cell-Br as a macroinitiator.42 Because of the hydrophobicity of the polyisoprene side chains and the hydrophilicity of the cellulose backbone, a typical two-phase morphology with rigid cellulose nanoscale domains dispersed in the polyisoprene matrix (Scheme 8B) was observed with two distinct Tg’s for these copolymers, as evidenced by dynamic mechanical analysis (DMA). However, the Cell-g-PI copolymers behaved like viscoelastic materials as the Tg of polyisoprene side chains was extremely low (around −60 °C). These phase-separated Cell-g-PI copolymers were utilized to construct elastomers mimicking the mechanical properties of human skin. Human skin exhibits unique nonlinear mechanical properties that are not easy to reproduce in artificial materials. It is soft at low strains and stiff at high strains, helping to protect internal organs and tissues from mechanical damage as well as to depress energy consumption during movements. The main structure contributing to the mechanical properties of human skin is the three-dimensional network formed by semirigid collagen fibers and soft elastin fibers. Cellulose is a
3.2. Polyisoprene-Grafted Cellulose as Elastomers
Isoprene, a monomer that can be obtained from biomass, was grafted from a cellulose backbone through ATRP. As shown in Scheme 8A, the graft copolymers cellulose-graf t-polyisoprene (Cell-g-PI) were synthesized via activator regenerated by 1770
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Accounts of Chemical Research Scheme 9. Overarching Summary and Future Directions on Biobased Elastomers
of polyisoprene-grafted cellulose and subsequent mechanical processing. By minimization of the cellulose content, high resilient elastomers with a well-defined network were also constructed. Although the chemical and assembled structures are not exactly the same as those of elastic proteins, the welldefined network, long and flexible polymer chains, and low intermolecular friction are accounted to explain their elastomeric similarity to elastic proteins. This approach may broaden the application of sustainable elastomers derived from biomass. In regard to various elastomers described above, the macromolecular architecture and composition play vital roles in dictating the properties of elastomers. Though there are parallel efforts on the integration of biomass into main-chain polymers, those with biomass in the polymer side chains should consider the balance of thermal and mechanical properties. A polymer with a lengthy or bulky group in the side chain (e.g., fatty acid or rosin) typically has a high entanglement molecular weight (Me). In order to have sufficient chain entanglement and thus reasonable mechanical strength, three strategies could be explored: (1) generation of ultrahigh molecular weight (at least 2 × Me) to allow enough chain entanglements, which requires robust polymerization techniques, such as ROMP; (2) control of the chain architecture, e.g., the use of a pentablock copolymer architecture, which can sufficiently circumvent the premature craze and fracture from low-entangled chains of the matrix;46,47 and (3) chemical modification of the side chains to force them to entangle, e.g., chemical cross-linking. However, the level of such modification has to be subtly controlled to avoid the formation of thermosets. Ideally, dynamic covalent bonding could be introduced, as recently utilized in many emerging applications.48−50 On the other hand, in some cases one can intentionally utilize the low chain entanglement to achieve particular properties. It has been well-demonstrated that low intermolecular friction partially dictates the high resilience of natural resilin.51 We recently developed a new approach for designing high resilient polymers with intrinsically low chain entanglement of side-chain fatty-containing polymers to reduce intermolecular friction, thus eliminating the need for additional force or agents.52 On top of fatty groups in the side chains, we introduced a dynamic Diels−Alder reaction to make mendable elastomers involving fatty chains.
skeletal molecule in plant cells, while collagen acts as a skeletal molecule in human skin. Chemically cross-linked polyisoprene (rubber) shows high elastic properties close to those of elastin. It was envisioned that three-dimensional networks with the proper combination of cellulose and polyisoprene might afford nonlinear elastic materials with mechanical properties that closely mimic those of human skin. Cross-linked Cell-g-PI copolymers were prepared via ARGET ATRP43 and subsequent activator regenerated by electron transfer atom transfer radical coupling (ARGET ATRC).44 DMA and other characterization results showed that Cell-g-PI copolymers self-assembled into a two-phase morphology: stiff cellulose domains dispersed in the elastic matrix of polyisoprene. After cyclic tensile processing of the chemically cross-linked Cell-g-PI, the cellulose domains were orientated along the tensile direction. The mechanically processed samples exhibited nonlinear elastic properties that closely mimic those of human skin.45 More importantly, the mechanical properties of skin-mimic elastomers can be finely controlled to mimic different types of skin simply by tuning of the cellulose content (ranging from 4.3 to 20.6 wt %). Moreover, high resilient elastomers (HREs) were obtained by minimizing the cellulose content in cross-linked Cell-g-PI, which has a well-defined network with long flexible polyisoprene chains (Figure 6A). After plasticization with mineral oil and cyclic tensile processing, cross-linked Cell-g-PI showed ultrahigh resilience, high ultimate strain, and relatively high stress. The stress−strain curves of HREs during step-cycle tensile tests showed negligible hysteresis (Figure 6B), demonstrating high resilient properties (Figure 6C).34
4. CONCLUSIONS AND PROSPECTIVES In this Account, we have discussed recent progress in exploring sustainable elastomers from renewable biomass. A few different approaches toward monofunctional monomers and polymers from plant oils have been developed for the design of ABA triblock copolymer TPEs. New macromolecular architectures toward next-generation TPEs using biomass as precursors have also been explored. Multigraft copolymers with a rigid backbone (cellulose or lignin) and elastic side chains behave like typical TPEs. This new approach could be generalized into other types of stiff substrates with elastic side chains. Humanskin-mimic elastomers were obtained by chemical cross-linking 1771
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Accounts of Chemical Research Though there has been a lot of progress, the properties of many biobased elastomers are still inferior to those of commercial petroleum-based elastomers. For example, SEBS thermoplastic elastomers (Kraton G) could have tensile strength and elongation at break of over 40 MPa and 800%, respectively. Compared with well-established thermal and mechanical processing of nonbiobased elastomers, challenges prevail in customizing new protocols for processing of biobased counterparts given their complexity of origins. We could pursue simplifying the compositions of biomass, which might require breakthroughs in chemical and enzymatic catalysis. In return, it may also improve and/or tune thermomechanical properties. One should be inspired by the successful development of thermoplastic poly(lactic acid), which by many standards is as good as many other petroleum-based plastics. Scheme 9 summarizes the state-of-the-art progress and possible future solutions toward next-generation biobased elastomers that not only compete with petroleum products but also provide additional benefits. Parallel efforts should be devoted to achieving highly entangled matrixes and hard minority domains, which are expected to provide tunable elasticity and toughness, respectively. Looking forward, robust, economical synthetic techniques are needed to make biomass-based monomers and elastomers before they can be in production on a commercial scale. One should pay attention to the sustainability of any newly innovated techniques or processes. We have to admit that great challenges still lie ahead before cost-effective and mechanically desirable elastomers from biomass become a success in the market. Exploratory hunting for functional elastomers, such as high resilient elastomers, skin-mimic elastomers, and self-healable elastomers, might need to be further conceptualized and performed for target applications.
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Prof. Dr. Chuanbing Tang received his B.S. from Nanjing University and Ph.D. from Carnegie Mellon University with Profs. Krzysztof Matyjaszewski and Tomasz Kowalewski. He was a postdoctoral scholar at the University of California Santa Barbara with Profs. Edward J. Kramer and Craig J. Hawker. He is currently a Distinguished Professor in Department of Chemistry and Biochemistry at the University of South Carolina. His research interests focus on organic polymer synthesis, sustainable polymers from biomass, metal-containing polymers, and polymers for biomedical applications and dielectric energy storage.
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ACKNOWLEDGMENTS C.T. acknowledges support from the National Science Foundation (DMR-1252611) and the United Soybean Board (Project 1540-612-6273). Z.W. thanks the National Natural Science Foundation of China for financial support (Grant 51603002).
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AUTHOR INFORMATION
Corresponding Authors
*(Z.W.) E-mail:
[email protected]. *(C.T.) E-mail:
[email protected]. ORCID
Chuanbing Tang: 0000-0002-0242-8241 Author Contributions §
Z.W. and L.Y. contributed equally to this work.
Notes
The authors declare no competing financial interest. Biographies Prof. Zhongkai Wang received his B.S. in Polymer Science and Engineering from Hefei University of Technology in 2009 and his Ph.D. in Material and Processing Engineering from the University of Science and Technology of China in 2014. After working with Dr. Chuanbing Tang as a postdoctoral researcher at the University of South Carolina, he joined the Department of Material Science and Engineering of Anhui Agriculture University in 2016. He is currently a Professor and leads a research group focusing on the development of polymeric materials from agriculture and forestry biomass. Dr. Liang Yuan received his B.S. in Chemistry (2009) and M.S. in Polymer Chemistry and Physics (2012), both from Nankai University, and completed his Ph.D. in Chemistry under the direction of Prof. Chuanbing Tang at the University of South Carolina in December 2016. His doctoral research was focused on developing sustainable polymeric materials from renewable biomass. 1772
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